In a previous, exceptionally off-topic (even for me) thread digression, an interesting discussion occurred concerning the relationship of natural selection to biodiversity between myself (obviously a proponent) and Elmer, who feels the concept of natural selection is vacuous - and hence meaningless in the context of biodiversity. I thought that this would be a good place to continue that conversation.
In order to avoid a descent into too scattershot or rambling a discussion, I’d like to try and frame the question a bit more clearly. In essence, there is a direct observation of a phenomenon that needs explaining.
Note: to avoid confusion, I’d like to clarify three terms that I will be using: biodiversity, guild composition, and selection pressure.
1. Biodiversity: When discussing biodiversity in this context, I will be focusing on
RAZD’s second definition: number of species in a given location. This is the definition most relevant to the example, and also the one I’m most familiar with because this is the scale (landscape) at which I work.
2. Guild composition (or composition
tout court): I’ll be using “composition” as a shorthand for a combination of quantitative measures: species diversity, functional distribution, relative abundance, etc. It is the differences observed in these measures which are most relevant to the example.
3. Natural selection: The non-random action of the suite of environmental factors (“selection pressures”) that affect a population. Both biotic and abiotic factors extant in the area under consideration are subsumed under “selection pressure”. If it becomes necessary, more detail can be examined.
Enough preamble, here is the observation that needs explaining:
Dung beetles (Coleoptera: Scarabaeinae) represent a speciose group of organisms with critical functional roles (decomposition, nutrient recycling) in many ecosystems. They are especially common in the tropics, in both the Old and New Worlds. At the landscape level, they form complex, multi-species guilds whose composition is differentiated by habitat type. The group appears highly sensitive to environmental change, and in fact guild composition can be directly mapped to different environmental gradients; for instance, that between disturbed habitat, ecotone (or transitional habitat) and mature forest. Any changes in habitat are directly reflected in changes in guild composition, whether the habitat change is “negative” (in the sense of anthropogenic impacts such as defaunation, introduction of exotic species like cattle, deforestation, etc), or “positive” (in the sense of habitat restoration, natural succession, etc). In fact, this guild’s composition is
so sensitive to environmental change and so tightly correlated to habitat integrity that it has been proposed as a valid surrogate for monitoring the health of the entire landscape - including all its other species assemblages and the ecosystem processes on which they depend. As the habitat changes, so too does the composition and structure of the guild.
So here’s the question before the board: What is the explanation (i.e., mechanism) which accounts for the tight correlation between guild structure/composition and habitat/habitat change with these organisms?
My answer: The observed differentiation can be accounted for by subtle differences in the environmental factors existing at each microsite along the given gradient. The interplay of these factors as they affect the specific species populations composing the guild at each microsite - subsumed under the term “natural selection” - is the mechanism by which guild composition changes in lockstep with changes in these environmental factors. These factors (“selection pressures”) either favor or inhibit the survival and reproduction of specific species which comprise the guild at each site, thus changing species diversity, relative abundance, or the other observed measures of guild structure and composition.
Elmer’s explanation is:??????????